Paricalcitol (1) is a synthetically manufactured vitamin D analog developed for the treatment of secondary hyperparathyroidism associated with chronic renal failure.
The known synthetic route to paricalcitol utilizes 25-hydroxyvitamin D2 (2) as the key starting material, but this compound is quite costly and has very limited commercial availability. As a result, alternative syntheses to compound 2 and its derivatives have been developed, mostly based on the functionalization of the Inhoffen-Lythgoe diol (3).

Such approaches involve a significant number of complex chemical transformations, with formation of several impurities, many of which carry through to the final active pharmaceutical ingredient (API). Of particular concern is the C20 epimer of paricalcitol, so similar in structure that even the HPLC analytical method listed in the United States Pharmacopeia fails to resolve both compounds.
U.S. Pat. Nos. 5,281,731 and 5,086,191 describe the purification of paricalcitol by preparative HPLC, but the cost and labor associated with this method make it undesirable for large scale manufacturing. More recently, published patent application US 2007/0093458 discloses crystallization procedures for the purification of paricalcitol. However, the examples given produce material of insufficient purity and/or in relatively low yields. Furthermore, the procedures described are volumetrically inefficient and many of them also require precise control of solvent ratios, volumes and temperatures.